Apparatus for Adjusting Focus Offset and Method Thereof

ABSTRACT

The present invention provides an apparatus of an optical disc drive for adjusting a focus error signal. The apparatus includes a focus offset determining unit and an adjusting module. The focus offset determining unit is utilized for determining a target focus offset corresponding to a target condition, and the adjusting module is coupled to the focus offset determining unit and utilized for receiving the focus error signal and adjusting the focus error signal with the target focus offset to generate an adjusted focus error signal.

BACKGROUND

The present invention relates to an apparatus applied to an optical discdrive with adjustable focus offset and a method thereof.

When an optical disc is accessed by a pickup head in an optical discdrive, the conventional servo system of the optical disc drive willcontrol the pickup head to emit a controlled laser emission to a properlocation on the optical disc. The optimum focus point of the laser power(emission) upon the optical disc varies with the type of pickup head,optical disc, etc. This is because different type of pickup heads willhave different structure of optical paths, and different types ofoptical discs will have different thicknesses. In other words, theminimum focus error (FE) signal will not guarantee that the currentfocus point is the optimum focus point upon a specific optical disc.Therefore, in the conventional focus control loop, a focus offset isadded to the focus error signal to generate an adjusted focus errorsignal, which then the adjusted focus error signal is transferred to acontroller. The controller tunes the position of the pickup head tooptimize the focus of the laser spot upon a data recording layer of theoptical disc. Accordingly, an optimized focus offset will be obtained toachieve an optimum reflected signal quality.

Furthermore, it is well-known that higher rotational speeds of theoptical disc result in decreased SNR performance, degrading the decodingaccuracy of the physical address and data of the optical disc. Thus, toincrease the reading quality of the optical disc, strength of the laserpower is dependent on the tangential velocity of the position of theoptical disc to be accessed. In other words, the focus offset should beadjusted when the laser power changes. If the same focus error isadopted and added to the focus error signal in a case where the laserpower changes in response to the tangent velocity change, the actualfocus of the laser spot is shifted from the optimum focus. Therefore, anovel scheme of dynamically calibrating the focus error is required tooptimize the focus of the laser spot for every position of the opticaldisc to be accessed.

SUMMARY

It is therefore one of the objectives of the claimed invention toprovide an optical disc drive for adjusting focus offset, and methodthereof.

According to an embodiment of the present invention, an apparatus of anoptical disc drive for adjusting a focus error signal is disclosed. Theapparatus comprises a focus offset determining unit and an adjustingmodule. The focus offset determining unit is utilized for determining atarget focus offset corresponding to a target condition, and theadjusting module is coupled to the focus offset determining unit andused for receiving the focus error signal and adjusting the focus errorsignal with the target focus offset to generate an adjusted focus errorsignal.

According to the embodiment of the present invention, a method for anoptical disc drive to adjust a focus error signal is disclosed. Themethod comprises determining a target focus offset corresponding to atarget condition, and receiving the focus error signal and adjusting thefocus error signal with the target focus offset to generate an adjustedfocus error signal.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an embodiment of an apparatus foradjusting a focus error signal when a pickup head utilizes a targetlaser power to access an optical disc according to the presentinvention.

FIG. 2 is a diagram illustrating the interpolation between the firstfocus offset and the second focus offset by referencing the first laserpower and the second laser power of the present invention.

FIG. 3 is a diagram illustrating the interpolation between the firstfocus offset, the second focus offset, and the third focus offset byreferencing the first laser power, the second laser power, and the thirdlaser power of the present invention.

FIG. 4 is a first flowchart illustrating the adjustment of the focuserror signal when the pickup head utilizes the target laser power toaccess the optical disc of FIG. 1.

FIG. 5 illustrates a second flowchart for adjusting the focus errorsignal when the pickup head utilizes the target laser power to accessthe optical disc of FIG. 1.

DETAILED DESCRIPTION

Please refer to FIG. 1. FIG. 1 illustrates a diagram for an embodimentof apparatus 100 of an optical disc drive 102, for adjusting a focuserror signal FE when a pickup head 101 utilizes a target laser power PWto access an optical disc 103 according to the present invention. Inaddition to the apparatus 100, the optical disc drive 102 furtherincludes a motor 108, a spindle 109, the pickup head 101, a servo system107, and a focus error (FE) signal generator 110. The apparatus 100comprises a focus offset calibration unit 104, a focus offsetdetermining unit 105, an adjusting module 106, an FE signal generator110, and a multiplexer 111. The FE signal generator 110 is able tooutput the focus error signal FE according to reflected light detectedby the pickup head 101. Since the generation of the focus error signalFE is well known, further description is omitted here for brevity. Thefocus offset calibration unit 104 is utilized to perform a first focusoffset calibration to obtain a first focus offset FO₁ when the pickuphead 101 utilizes a first laser power PW₁ to access the optical disc103, and to perform a second focus offset calibration to obtain a secondfocus offset FO₂ when the pickup head 101 utilizes a second laser powerPW₂ to access the optical disc 103. It should be noted that the firstlaser power PW₁ is different from the second laser power PW₂ in thisembodiment. The focus offset determining unit 105 is coupled to thefocus offset calibration unit 104, and is able to determine a targetfocus offset corresponding to a target condition, where the targetcondition can be a target laser power, a target position on the opticaldisc 103, or a target tangential velocity associated with a position onthe optical disc 103, etc. For instance, the focus offset determiningunit 105 can calculate a target focus offset FO corresponding to thetarget laser power PW according to at least the first focus offset FO₁,the second focus offset FO₂, the first laser power PW₁, the second laserpower PW₂, and the target laser power PW. There are many ways for thefocus offset determining unit 105 to calculate the target focus offsetFO corresponding to the target laser power PW. For example, the focusoffset determining unit 105 can derive the target focus offset FO byperforming an interpolation or extrapolation according to at least thefirst focus offset FO₁ and second focus offset FO₂ or by obtainingmultiple focus offsets to determine an equation based on these focusoffsets according to curve fitting. The target focus offset FO can alsobe obtained from a lookup table. The first focus offset FO₁ and thesecond focus offset FO₂ are obtained by the focus offset determiningunit 105 before the target focus offset is determined; the first focusoffset FO₁ and the second focus offset FO₂ can be obtained offline oronline. The adjusting module 106 is able to receive the focus errorsignal FE and to adjust the focus error signal FE with the target focustarget offset to generate an adjusted focus error signal FE′.

Furthermore, the servo system 107 is coupled to the focus offsetdetermining unit 105, the adjusting module 106, the pickup head 101, andthe motor 108, in which the motor 108 is coupled to a spindle 109 of theoptical disc 103 for driving the spindle 109 to rotate in differentmodes. The multiplexer 111 is controlled by a calibration flag, which isoutputted from the focus offset determining unit 105. When thecalibration flag is bit ‘1’ and the optical disc drive 102 enterscalibration mode, the focus offset calibration unit 104 controls theprocedure of obtaining the first focus offset FO₁ and the second focusoffset FO₂, which are to be transmitted to the focus offset determiningunit 105. In other words, the focus offset calibration unit 104calculates respective focus offsets corresponding to differentconditions, e.g. different laser powers, different positions on theoptical disc 103, or tangential velocities of the different positions onthe optical disc 103. After the respective focus offsets are calculated,the focus offset calibration unit 104 records these focus offsets, whichare to be used by the focus offset determining unit 105 for determiningthe target focus offset FO corresponding the target condition. When thecalibration flag is bit ‘0’ and the optical disc drive 102 enters normalmode, the focus offset determining unit 105 will determine the targetfocus offset FO corresponding to the target condition according to therecorded focus offsets and their corresponding conditions; for example,the focus offset determining unit 105 calculates the target focus offsetFO according to the target laser power PW, first laser power PW₁, secondlaser power PW₂, first focus offset FO₁, and the second focus offsetFO₂.

On the other hand, the laser power of the pickup head 101 isproportional to the linear (tangential) velocity of the accessedposition on the optical disc 103. Therefore, when the apparatus 100operates in the CAV (Constant angular velocity) mode (or partialconstant angular velocity mode, PCAV), which the servo system 107controls the motor 108 to drive the velocity at the position of theinner radius of the optical disc 103, it is the slowest. The linear(tangential) velocity at the position of the outer radius of the opticaldisc 103 is the fastest, and thus the laser power of the pickup head 101at the inner radius of the optical disc 103 is the lowest and the laserpower at the outer radius of the optical disc 103 is the highest. Inorder to optimize the target laser power PW to focus on the optical disc103, for example, the focus offset determining unit 105 will firstperform an interpolation to obtain the target focus offset FO byreferencing the first focus offset FO₁, the second focus offset FO₂, thefirst laser power PW₁, the second laser power PW₂, and the target laserpower PW. Meanwhile, the optical disc 103 is loaded into the opticaldisc drive 102. Please refer to FIG. 2. FIG. 2 is a diagram illustratingthe interpolation between the first focus offset FO₁ and the secondfocus offset FO₂ by referencing the first laser power PW₁ and the secondlaser power PW₂ according to an embodiment of the present invention.

In the CAV mode, to obtain the first focus offset FO₁, the second focusoffset FO₂, the first laser power PW₁, the second laser power PW₂, thepickup head 101 first utilizes the first laser power PW₁ to access afirst position X₁ (e.g. on the inner radius of the optical disc 103 inFIG. 1) on the optical disc 103, where the first position X₁ correspondsto a first tangent velocity V₁. The focus offset calibration unit 104will determine the first focus offset FO₁ and then adjust the focuserror signal FE by the first focus offset FO₁. The adjusted focus errorsignal FE′=FE−FO₁ is transferred to the servo system 107, in which theservo system 107 controls the first laser power PW₁ of the pickup head101 according to the adjusted focus error signal FE′. Accordingly, thepickup head 101 can emit the first laser power PW₁ to the optimal focuspoint on a desired recording layer of the optical disc 103 forreading/writing data thereof. Then, when the pickup head 101 utilizesthe second laser power PW₂ to access a second position X₂ (e.g. on theouter radius of the optical disc 103 in FIG. 1) on the optical disc 103,where the second position X₂ corresponds to a second tangent velocityV₂, the focus offset calibration unit 104 will determine the secondfocus offset FO₂ and then adjust the focus error signal FE by the secondfocus offset FO₂. The adjusted focus error signal FE′=FE−FO₂ istransferred to the servo system 107, in which the servo system 107controls the second laser power PW₂ of the pickup head 101 according tothe adjusted focus error signal FE′. Accordingly, the pickup head 101can emit the second laser power PW₂ to the optimal focus point on adesired recording layer of the optical disc 103 for reading/writing datathereof. Please note that the above operation is dominated by awell-known focus control loop, and hence a detailed description isomitted here. Additionally, in this embodiment, the selection of thetest positions X₁ and X₂ is only for illustrative purposes. The presentinvention is not limited to use the test laser power to access these twotest positions X₁ and X₂.

Please refer to FIG. 2 and FIG. 1, the x-axis represents the laser powerand the y-axis represents the focus offset, which can be viewed as thefocus offset between FO₁ and FO₂ this is obtained through interpolationwith respect to the laser power between PW₁ and PW₂. Furthermore, thefocus offset determining unit 105 utilizes equation (1) to perform theinterpolation to determine the target focus offset FO corresponding tothe target laser power PW.

FO=FO ₁+((FO ₂ −FO ₁)/(PW ₂ −PW ₁))(PW−PW ₁)   (1)

Accordingly, whenever the pickup head 101 accesses any position betweenthe first position X₁ and the second position X₂, by using a laser powerbetween the first laser power PW₁ and the second laser power PW₂, thefocus offset determining unit 105 will provide the corresponding focusoffset to the multiplexer 111 according to equation (1). That is, thefocus offset determining unit 105 can determine one target focus offsetcorresponding to any target condition, e.g. a target position on theoptical disc 103. Additionally, since there exists a mappingrelationship between positions on the optical disc 103 and thetangential velocities of the positions, the focus offset determiningunit 105 can also determine a target focus offset FO corresponding to atarget tangential velocity of any position on the optical disc 103. Themultiplexer 111 then directly outputs the target focus offset calculatedby the focus offset determining unit 105 to the adjusting module 106. Itshould be noted that the required parameters FO₁ and FO₂, which arereferenced by the focus offset determining unit 105, are determined bythe focus offset calibration unit 104. In the CAV mode, because thelaser power of the pickup head 101 is proportional to the positionradius R of the optical disc 103, the focus offset determining unit 105can utilize the position radius R to proceed with interpolation, inother words, equation (1) can be changed as below:

FO=FO ₁+((FO ₂ −FO ₁)/(R ₂ −R ₁))(R−R ₁)   (2)

Accordingly, whenever the pickup head 101 accesses any position having aposition radius between the first position radius R₁ shown in FIG. 1 andthe second position radius R₂ shown in FIG. 1, the focus offsetdetermining unit 105 will provide the corresponding focus offset to themultiplexer 111, using a laser power between the first laser power PW₁and the second laser power PW₂, and according to equation (2).Therefore, the present invention is capable of adding different targetfocus error values to the focus error signal FE of each position betweenthe first position X₁ and the second position X₂ on the optical disc103.

As mentioned above, the laser power for accessing a specific discposition changes as the tangent velocity of the specific disc positionchanges. In addition, it is well known that in a CAV mode, the tangentvelocity is proportional to the position radius. Briefly summarized,according to the present invention, the pickup head 101 utilizes thefirst laser power PW₁ to access a first position X₁ on the optical disc103, where the first position X₁ corresponds to a first tangent velocityV₁ The pickup head 101 utilizes the second laser power PW₂ to access asecond position X₂ on the optical disc 103, where the second position X₂corresponds to a second tangent velocity V₁. The pickup head 101 alsoutilizes the target laser power PW to access a target position X on theoptical disc 103 where the target position X corresponds to a targettangent velocity V, and the focus offset determining unit 105 performsthe interpolation to obtain the target focus offset FO by referencingthe first focus offset FO₁, the second focus offset FO₂, the firsttangent velocity V₁, the second tangent velocity V₂, and the targettangent velocity V.

Please note that, in order to prevent nonlinearity of the optical disc103, the present invention can also perform the focus error signal FEcalibration according to radius intervals of the optical disc 103.Therefore, in another embodiment of the present invention, the focusoffset calibration unit 104 further performs a third focus offsetcalibration to obtain a third focus offset FO₃ when the pickup head 101utilizes a third laser power PW₃ to access the optical disc 103. Thesecond laser power PW₂ is between the first laser power PW₁ and thethird laser power PW₃. Please refer to FIG. 3, FIG. 3 is a diagramillustrating the interpolation between the first focus offset FO₁, thesecond focus offset FO₂, and the third focus offset FO₃ by referencingthe first laser power PW₁, the second laser power PW₂, and the thirdlaser power PW₃ of the present invention. When the target laser power PWfalls between the first laser power PW₁ and the second laser power PW₂,the focus offset determining unit 105 calculates the target focus offsetFO corresponding to the target laser power PW according to the firstfocus offset FO₁ and the second focus offset FO₂ (equation (1) orequation (2)). When the target laser power PW falls between the secondlaser power PW₂ and the third laser power PW₃, the focus offsetdetermining unit 105 calculates the target focus offset FO correspondingto the target laser power PW, the second focus offset FO₂, and the thirdfocus offset FO₃; related description is illustrated in equation (3):

FO=FO ₂+((FO ₃ −FO ₂)/(PW ₃ −PW ₂))(PW−PW ₂)   (3)

or equation (4):

FO=FO ₂+((FO ₃ −FO ₂)/(R ₃ −R ₂))(R−R ₂)   (4)

Furthermore, please note that, the accessing operation of the pickuphead 101 in the above mentioned embodiments includes: writing data tothe optical disc 103, or reading data from the optical disc 103. Thatis, the disclosed focus error calibration can be applied when datawriting or data reading is enabled. Additionally, using two radiusintervals is only meant for illustrative purposes. Other embodimentsinvolving segmenting the optical disc 103 into more than two radiusintervals are possible.

Please refer to FIG. 4. FIG. 4 illustrates a first flowchart foradjusting the focus error signal FE when the pickup head 101 utilizesthe target laser power PW to access the optical disc 103 of FIG. 1. Themethod includes the steps as listed below:

-   -   Step 401: Load the optical disc 103.    -   Step 402: Perform the first focus offset calibration to obtain        the first focus offset FO₁ when the pickup head 101 utilizes the        first laser power PW₁ to access the optical disc 103, and        perform the second focus offset calibration to obtain the second        focus offset FO₂ when the pickup head 101 utilizes the second        laser power PW₂ to access the optical disc 103.    -   Step 404: Perform interpolation to obtain the target focus        offset FO by referencing the first focus offset FO₁, the second        focus offset FO₂, the first laser power PW₁, and the second        laser power PW₂.    -   Step 406: Adjust the focus error signal FE with the target focus        offset FO.

Please note that, in step 404, because the laser power of the pickuphead 101 is proportional to the tangent velocity of the accessedposition on the optical disc 103. Therefore, when the optical disc 103operates in the CAV (Constant angular velocity) mode, the first laserpower PW₁ and the second laser power PW₂ can be replaced with thecorresponding first position X₁ and second position X₂, respectively.These are represented by equation (1) and the equation (2),respectively.

Please refer to FIG. 5. FIG. 5 illustrates a second flowchart foradjusting the focus error signal FE with the target focus offset FO whenthe pickup head 101 utilizes the target laser power PW to access theoptical disc 103 of FIG. 1. The method includes the steps as below:

-   -   Step 501: Load the optical disc 103.    -   Step 502: Perform the first focus offset calibration to obtain        the first focus offset FO₁ when the pickup head 101 utilizes the        first laser power PW₁ to access the optical disc 103, perform        the second focus offset calibration to obtain the second focus        offset FO₂ when the pickup head 101 utilizes the second laser        power PW₂ to access the optical disc 103, and perform the third        focus offset calibration to obtain the third focus offset FO₃        when the pickup head 101 utilizes the third laser power PW₃ to        access the optical disc 103.    -   Step 503: Identify the target laser power PW. If the target        laser power PW falls between the first laser power PW₁ and the        second laser power PW₂, go to step 504; and if the target laser        power PW falls between the second laser power PW₂ and the third        laser power PW₃, go to step 505.    -   Step 504: Perform interpolation to obtain the target focus        offset FO by referencing the first focus offset FO₁, the second        focus offset FO₂, the first laser power PW₁, and the second        laser power PW₂. Then go to step 506.    -   Step 505: Perform interpolation to obtain the target focus        offset FO by referencing the second focus offset FO₂, the third        focus offset FO₃, the second laser power PW₂, and the third        laser power PW₃.    -   Step 506: Adjust the focus error signal FE with the target focus        offset FO.

Please note that in step 504, because the laser power of the pickup head101 is proportional to the tangent velocity of the accessed position onthe optical disc 103. Therefore, when the optical disc 103 operates inCAV (Constant angular velocity) mode, the first laser power PW₁ and thesecond laser power PW₂ can be replaced with the corresponding firstposition X₁ and second position X₂ respectively. These are alsorepresented by equation (1) and the equation (2). Similarly, in step505, the second laser power PW₂ and the third laser power PW₃ can bereplaced with the corresponding second position X₂ and third positionX₃, which are also represented by equation (3) and the equation (4),respectively.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. An apparatus of an optical disc drive for adjusting a focus errorsignal, comprising: a focus offset determining unit for determining atarget focus offset corresponding to a target condition; and anadjusting module, coupled to the focus offset determining unit, forreceiving the focus error signal and adjusting the focus error signalwith the target focus offset to generate an adjusted focus error signal.2. The apparatus of claim 1, wherein the target condition is a targetlaser power, a target position on an optical disc, or a targettangential velocity associated a position on the optical disc.
 3. Theapparatus of claim 1, wherein the target condition is a target laserpower, and the focus offset determining unit obtains at least a firstfocus offset corresponding to a first laser power and a second focusoffset corresponding to a second laser power before determining thetarget focus offset.
 4. The apparatus of claim 3, wherein the focusoffset determining unit performs an interpolation/extrapolation toobtain the target focus offset according to the first focus offset, thesecond focus offset, the first laser power, the second laser power, andthe target laser power.
 5. The apparatus of claim 3, wherein the focusoffset determining unit obtains the target focus offset from a lookuptable according to the first focus offset, the second focus offset, thefirst laser power, the second laser power, and the target laser power.6. The apparatus of claim 3, wherein the focus offset determining unitobtains the target focus offset corresponding to the target laser powerby an equation based on the first focus offset, the second focus offset,the first laser power, and the second laser power according to a curvefitting.
 7. The apparatus of claim 3, further comprising: a multiplexer,coupled to the focus offset determining unit and the adjusting module;and a focus offset calibration unit, coupled to the multiplexer and thefocus offset determining unit, for generating the first and second focusoffsets according to the first and second laser powers respectively;wherein under calibration mode the focus offset calibration unitgenerates the first and second focus offsets by respectively adjustingdifferent focus offsets outputted to the adjusting module via themultiplexer; and under normal mode the focus offset determining unitdetermines the target focus offset according to the first and secondfocus offsets generated by the focus offset calibration unit.
 8. Amethod for an optical disc drive to adjust a focus error signal,comprising: determining a target focus offset corresponding to a targetcondition; and receiving the focus error signal and adjusting the focuserror signal with the target focus offset to generate an adjusted focuserror signal.
 9. The method of claim 8, wherein the target condition isa target laser power, a target position on an optical disc, or a targettangential velocity associated a position on the optical disc.
 10. Themethod of claim 8, wherein the target condition is a target laser power,and the method further comprises: obtaining at least a first focusoffset corresponding to a first laser power and a second focus offsetcorresponding to a second laser power before determining the targetfocus offset.
 11. The method of claim 10, wherein the step ofdetermining the target focus offset comprises: performing aninterpolation/extrapolation to obtain the target focus offset accordingto the first focus offset, the second focus offset, the first laserpower, the second laser power, and the target laser power.
 12. Themethod of claim 10, wherein the step of determining the target focusoffset comprises: obtaining the target focus offset from a lookup tableaccording to the first focus offset, the second focus offset, the firstlaser power, the second laser power, and the target laser power.
 13. Themethod of claim 10, wherein the step of determining the target focusoffset comprises: obtaining the target focus offset corresponding to thetarget laser power by an equation based on the first focus offset, thesecond focus offset, the first laser power, and the second laser poweraccording to a curve fitting.